The effect of superhelical stress to induce new global secondary structures and different tertiary conformations in circular DNAs is studied using a variety of techniques. Secondary structure governs the torsional rigidity of the filament and is studied via the time-resolved fluorescence polarization anisotropy of intercalated ethidium, dynamic light scattering and circular dichroism. Tertiary structure is monitored by both gel electrophoresis and dynamic and static light scattering. The existence and number of distinct secondary structures induced by changes in buffer, and their responses to temperature, intercalator concentration, and pH are examined. The influence of a 48 bp inverted repeat on both secondary and tertiary structures is also investigated. The effects of certain proteins, including single-strand binding protein and CAO protein from E. coli and antibodies specific for Z-helix, on the secondary and tertiary structures of supercoiled DNAs are also investigated.